It is known that, in order to effect this detection, receptacles are used containing a layer of growth medium for receiving micro-organisms coming from the environment which it is wished to monitor, the receptacle next being put to incubate at the required temperature and for the required time to enable the micro-organisms received to develop in the form of colonies visible to the naked eye, so that they can be counted and identified.
The invention aims to improve the conditions for the development, and therefore the counting and identification, of the organisms received on the layer of growth medium.
To this end it proposes a method for detecting micro-organisms, characterized in that it includes the use of a culture receptacle having a layer of growth medium, a first end surface of which is adjacent a first chamber and the second end surface of which, opposite to the first, is adjacent a second chamber, and in that it includes a step of causing a higher pressure in said first chamber than in said second chamber.
The pressure difference then existing between the two end surfaces of the layer of the growth medium makes it possible, where its moisture level has become heterogeneous, to produce a diffusion of liquid inside this layer having the effect of rehomogenizing it, in particular at the surface receiving the micro-organisms.
The invention thus makes it possible to eliminate, or to very greatly reduce, the heterogeneity of the layer of growth medium, which is detrimental to the development of the micro-organisms and consequently to the counting and identification thereof.
According to preferred features, said receptacle is provided so that the first chamber is fluid tight, said step of causing a higher pressure including a step of closing this fluid tight chamber.
The step of causing a higher pressure is particularly easy and convenient to implement, the higher pressure resulting from a certain degree of compression of the volume of air trapped when the chamber is closed and/or the expansion of the air trapped in the chamber when the temperature rises due to placing in an incubator, where the fluid tight chamber has been closed in an environment at a temperature below that of the incubator.
According to other preferred features, the method according to the invention includes, prior to said step of causing a higher pressure, a step of striking said second end surface of the layer of growth medium with air jets.
Such a step of striking with air jets takes place in some air analysis apparatus in order to impact the micro-organisms present in the air on the layer of growth medium.
Causing a higher pressure is particularly advantageous after such a striking step as the air jets create craters in the layer of growth medium and dry the growth medium. By pressurizing of the surface opposite to the one which has been struck by the air jets, one causes a diffusion of liquid towards the dried surface, and therefore the rehomogenization of the layer of growth medium.
According to other preferred features, the method according to the invention includes a step of depositing a predetermined volume of rehydrating solution on said first end surface of the layer of growth medium prior to said step of causing a higher pressure. Such an addition of rehydrating solution, which will diffuse in the layer of growth medium by virtue of the pressurization, is particularly advantageous where the layer of growth medium has undergone a certain degree of drying, the existence of the higher pressure allowing excellent diffusion of the solution, appreciably greater than that able to be obtained simply by the effect of gravity.
The rehydrating solution can simply be water, or water with nutritive substances and/or specific dyes added. The diffusion procured by the higher pressure distributes these nutritive substances and/or dyes in the growth medium.
Thus, using a receptacle containing initially standard growth medium, it is possible to modify its properties in order to obtain a specific growth medium enabling certain predetermined micro-organisms to be counted and identified selectively.
According to other preferred features, the culture receptacle used has a grid coated with the layer of growth medium, which is oriented parallel to said grid. The presence of such a grid is particularly advantageous for providing a frame for the layer of growth medium (it is not possible to count on a holding of the layer of growth medium by means of an end surface, since each of these end surfaces must give onto a chamber).
The invention also relates, in a second aspect, to a cartridge for culturing micro-organisms, having a body with a grid and an annular wall oriented transversely to said grid and surrounding it, and having a layer of growth medium, preferably a gelled or self supporting medium such as agar; oriented parallel to said grid and coating it, characterized in that, in order to be suitable for the implementation of the method as disclosed above, said body has, between said annular wall and said grid, a solid annulus having towards the inside an extra thickness part forming a ridge.
This extra thickness ridge, because it is present towards the inside, remains coated by the layer of growth medium, even when the latter has undergone a certain shrinkage which has had the effect of separating its circumference from the annular wall of the body.
Given that the ridge remains coated and the annulus is solid, there is no possible passage between the circumference of the layer of growth medium and the body, so that the spaces situated inside the body respectively on each side of the layer of growth medium are separated in a fluid tight fashion, which makes possible the difference in pressure between the two end surfaces of layer of growth medium.
According to preferred features, for reasons of simplicity and convenience of manufacture of the cartridge and implementation of the method:
(a) the cartridge has a removable base adapted to be fitted onto said body in order to delimit an fluid tight chamber onto which a first end surface of said layer of growth medium is located; and preferably said base is fitted externally onto said annular wall of said body and has an internal surface coming into contact with the end surface of said annular wall situated on the same side as the base; and/or preferably said body and said base are made from molded plastic, said base being more flexible than said body; and/or PA1 (b) the cartridge has a removable cover adapted to be fitted onto said body in order to delimit a chamber in communication with the outside onto which a second end surface of said layer of growth medium gives; and preferably said cover has at least one hole allowing communication between the inside and outside of the cover when the latter is fitted onto said body; and/or preferably said cover is fitted externally onto said annular wall of said body and has an internal surface coming into contact with the end surface of said annular wall situated on the same side as the cover; and/or preferably said body and said cover are made from molded plastic, said cover being more flexible than said body.
According to other preferred features, in order to facilitate the counting of the colonies once the culturing has been effected, said grid has square meshes, and optionally is in a color contrasting with the layer of growth medium.
The disclosure of the invention will now be continued with the description of an example embodiment, given below for illustration and is not meant to be limiting in any manner.